1. Field of the Invention
The present invention relates to smoke detectors employing detection cells of the ionization type, and to the associated electrical circuitry for operation of the detection cell and for sensing the electrical changes which occur in the presence of airborne combustion products:
2. Description of the Prior Art
A smoke detection cell of the ionization type and circuits for dc operation of the detection cell are described in patent applications of Robert J. Salem, Ser. No. 630,204, filed Nov. 10, 1975, entitled "Smoke Simulating Test Apparatus for Smoke Detectors" and Ser. No. 630,202, filed Nov. 10, 1975, entitled "High Gain Sensing and Switching Means for Smoke Detectors", and assigned to the Assignee of the present application.
A smoke detection cell of the ionization type suitable for use in the present application is described in said applications. It includes an alpha particle radiation source, such as a small quantity of Americium 241, in a measuring chamber having positive and negative electrodes. The measuring chamber ionizes the air between the electrodes, permitting the flow of a small electrical current when a dc voltage is applied across the electrodes. When airborne products of combustion (smoke) enter the measuring chamber, an increase in resistance to the flow of current is observed. The resulting change in the electrical conductivity of the measuring chamber is sensed and used to trigger an alarm when the change exceeds a given quantity. The latter quantity is selected to correspond to a level of smoke or aerosols within the measuring chamber representing a dangerous condition.
A smoke detector which provides for ac operation for the detection cell is described in the patent application of Joseph P. Hesler, Ser. No. 728,524, filed Oct. 1, 1976, entitled "Smoke Detector", and assigned to the Assignee of the present application. In that application, the change in detection cell impedance in the presence of airborne combustion products alters the operating frequency of the network. The frequency change is sensed to actuate the alarm. The network utilizes MOS-FET devices as the active circuit elements.
Electrical conductivity of the measuring chamber is sensed in said patent applications by measurement of the voltage across the measuring chamber. In the open-circuit voltage measurement process, which the Hesler application employs, the measurement network should retain a high impedance in respect to that of the detection cell. With FET devices this is typically 10.sup.12 ohms, a figure which is one and two orders greater than the impedance (4 .times. 10.sup.10 ohms) of the cell. Under ideal conditions, this factor is quite adequate for accurate measurements. In the presence of moisture in the air or surface contamination, these impedances may change enough to affect the measurement accuracy. If an ac measurement is made, the problem of maintaining small capacitances at a stable value may also be present.